Drug delivery devices have application where regular injection by persons without formal medical training occurs. This may be increasingly common among patients having diabetes where self-treatment enables such patients to conduct effective management of their disease. In practice, such a drug delivery device allows a user to individually select and dispense a dose of a medicament, which may be a fixed dose or a user-variable dose. The medicament is usually contained within a cartridge. There are basically two types of drug delivery devices: resettable devices (i.e., reusable) and non-resettable (i.e., disposable). For example, disposable drug delivery devices are supplied as self-contained devices. Such self-contained devices contain pre-filled cartridges which may not be removed from these devices and replaced without destroying the device itself. In contrast, the reusable devices comprise a mechanism with which an empty or nearly empty cartridge may be replaced by a full cartridge. Consequently, such disposable devices need to have a resettable dose setting mechanism. The present disclosure is in general applicable for both types of devices, i.e. for disposable devices as well as for reusable devices.
In order to dispense the medicament from the cartridge the drug delivery device usually comprises a dose mechanism which drives a bung (piston) of the cartridge in the distal direction forcing the medicament contained in the cartridge out of a needle which is connected with a distal opening of the cartridge. In particular, the load provided by the dose mechanism to the bung is transmitted by a rod-like driving element (drive train), in the following referred to as piston rod. The piston rod is axially displaceable and applies a load in the distal direction along or parallel to the longitudinal axis of the drug delivery device to the cartridge bung, wherein the load is generated by the dose mechanism.